The invention pertains to an excavating tool such as, for example, a rotary drill bit, including the cutting insert therefor, and a method of drilling using the rotary drill bit, wherein the bit is useful for drilling through various earth strata. More specifically, the invention pertains to a roof drill bit, including the cutting insert therefor, and a method for using the roof drill bit, wherein the bit is useful for drilling bore holes in an underground mine.
The expansion of an underground mine, such as for example, a coal mine, requires digging a tunnel. Initially, this tunnel has an unsupported roof. Because the roof is not supported, there is an increased chance for a mine cave that, of course, adds to the hazards of underground coal mining. Furthermore, an unsupported roof is susceptible to rock and debris falling from the roof. Falling rock and debris can injure workers as well as create hazardous clutter on the floor of the tunnel.
In order to support and stabilize the roof in an underground tunnel, bore holes are drilled in the roof, i.e., earth strata. The apparatus used to drill these holes comprises a drill with a long shaft, i.e., drill steel, attached to the drill. A roof bit is detachably mounted to the drill steel at the distal end thereof. The roof bit is then pressed against the roof, and the drilling apparatus operated so as to drill a bore hole in the roof. The bore holes extend between about two feet and about twenty feet into the roof depending upon the particular situation. The typical rate of rotation is between about 100 revolutions per minute (rpm) to about 800 rpm, and the typical thrust is between about 1000 pounds to about 10,000 pounds for a time sufficient to drill the desired hole in the earth strata.
Roof support members, such as roof panels, are then attached to roof bolts. In one alternative procedure, these bore holes are filled with resin and roof bolts are fixed within the bore holes. In another alternative procedure, the roof bolts use mechanical expander shells to affix the roof bolts in the bore holes. The end result of using either procedure is a roof which is supported, and hence, is of much greater stability than the unsupported roof. This reduces the hazards associated with underground mining. The roof bolting process is considered to be an essential underground mining activity.
Roof bolting accounts for the largest number of lost time injuries in underground mining. During the roof bolting process, the roof is unsupported so that it does not have optimum stability. Furthermore, the roof bolting process exerts stresses on the roof so as to further increase the safety hazards during the roof bolting process. Thus, a decrease in the overall time necessary to bore holes reduces the time it takes to complete the roof bolting process. This is desirable since it contributes to the overall speed, efficiency and safety of the roof bolting process.
While there may be additional ways to decrease the overall time to complete the drilling of the necessary bore holes, one way is to use a roof drill bit that has a longer useful life so as to decrease the number times a roof drill bit must be replaced during the roof bolting process. Another way to decrease the overall time to complete the roof bolting process is to use a roof drill bit that drills the boreholes faster.
A roof drill bit typically comprises a steel bit body that attaches to a drill steel. The bit body has an axial forward end to which a cutting insert is affixed typically by brazing. The cutting insert is the component of the roof drill bit that typically has the greatest impact on the useful life of the roof drill bit and on the speed at which the roof drill bit drills holes.
Hence, heretofore, persons have developed cutting inserts for roof drill bits wherein the cutting inserts had various geometries. For example, U.S. Pat. No. 4,342,368 to Denman discloses a cutting insert for a roof drill bit. This cutting insert has a leading face and a frontal face that intersect to form a cutting edge. This cutting insert further includes a cut-out.
As another example, U.S. Pat. No. 4,787,464 to Ojanen discloses a cutting insert for a roof drill bit. This cutting insert has a leading face inclined at a constant angle with respect to the axis of rotation. The cutting insert also has a frontal face with a variable relief angle decreasing with increasing radial distance from the axis at its radial distal edge.
As still another example for a cutting insert for a roof drill bit, U.S. Pat. No. 6,595,305 to Dunn et al., shows a roof drill bit that has a cutting insert at the axial forward end thereof. The cutting insert has a trio of cutting edges.
In severe drilling conditions or in laminated geological conditions, a roof drill bit that uses a thicker cutting insert (e.g., a cutting insert that has a thickness equal to about 0.250 inches (6.35 millimeters)) typically will exhibit less breakage as compared to a roof drill bit that uses a thinner cutting insert (e.g., a cutting insert that has a thickness equal to about 0.180 inches (4.57 millimeters)). A roof drill bit that uses a thicker cutting insert will provide one way to decrease the overall time to complete the roof bolting process in view of the reduction in the occurrences of breakages. However, the use of a roof drill bit that uses a thicker cutting insert typically experiences a reduction in the penetration rate, and hence, while there is a decrease in the breakage of the cutting inserts, there is a corresponding reduction in the overall drilling speed.
It therefore becomes apparent that it would be desirable to provide an improved roof drill bit that facilitates the prompt completion of the roof bolting process. It is also apparent that it would be desirable to provide an improved roof drill bit that has a longer useful life. It is further apparent that it would be desirable to provide an improved roof drill bit that has an increased penetration rate. Finally, it is apparent that it would be desirable to provide an improved roof drill bit that has both a longer useful life and an increased penetration rate.